1. Field of the Invention
This invention relates to a method and apparatus for determining stress underground and is more particularly concerned with a vane stress sensor and process of using the same.
2. Description of the Prior Art
In the past, it has been extremely difficult to measure the stress in subsurface soil. Simultaneous direction oriented measurements have not, to our knowledge, been undertaken.
Applicant is, however, aware of U.S. Pat. No. 3,499,320.
In 12 Canadian Geotechnical Journal, Pages 143, 144 the prior art Glotzl earth pressure cell is described. This Glotzl measuring system is described as having a steel cell containing and confining oil adjacent a membrane. Air under pressure deflects the membrane toward the oil to open the valve so that the air is then returned, through a second tube when the membrane is deflected by pressure equalization. Another Glotzl type device is the pore water pressure cell which includes a flexible membrane within a porous cell. Water flows from the exterior into the cell to exert pressure to tend to close the membrane diaphragm. When air pressure on the other side of the membrane diaphragm overcomes this pressure, the valve is opened to return the air. Bubbles in the water container indicate that the valve has been opened and thus the air pressure gauge is read as an indication of the corresponding pressure adjacent to the cell.
The prior art instruments described above are expensive, difficult to install, give only very approximate readings, read only lateral stress perpendicular to the cell, give no simultaneous readings, and do not give stress readings in a vertical direction. The present invention, which overcomes these defects, provides a device and a process by which stress in several directions can be simultaneously or selectively measured.